Asynchrony, fragmentation, and scale determine benefits of landscape heterogeneity to mobile herbivores

Abstract

Understanding the ways that resource heterogeneity shapes the performance of individuals and the dynamics of populations offers a central challenge in contemporary ecology. Emerging evidence shows that herbivores track heterogeneity in nutritional quality of vegetation by responding to phenological differences in plants, differences that result from spatial and temporal variation in conditions favoring plant growth. Theory predicts that when spatial variation in temperature, nutrients, or moisture results in spatially asynchronous pulses of plant growth, herbivores are able to prolong the period during which they have access to forage of peak nutritional value. Although this idea has substantial support from observational and modeling studies, it has not been examined experimentally. We hypothesized that access to asynchronous resources enhances nutritional status and growth of herbivores and that the magnitude of this effect depends on the scale of access relative to the grain of resources. We tested these hypotheses in mesocosm experiment using the migratory grasshopper, Melanoplus sanguinipes, feeding on young wheat and protein-rich bran as a model system. We demonstrated access to asynchronous pulses in resources enhanced the efficiency of use of high quality resource use and increased growth of individuals by 13%. Disruption of this mechanism when landscapes were fragmented lowered efficiency of resource use and caused growth of individuals to decline by 15%. However, the strength of the effects of fragmentation on herbivore performance depended on the spatial extent of fragmentation relative to the spatial and temporal grain of resource emergence. Our findings add experimental support to modeling and observational studies that have linked herbivore performance to spatial and temporal variation in plant phenology. We also offer evidence that fragmentation can impair herbivore performance, even when the total amount and quality of resources on landscapes remains unchanged.

Asynchronous vegetation phenology enhances winter body condition of a large mobile herbivore

“…Emerging evidence suggests that limits set by ANPP are modified by the spatial pattern and timing of plant growth. In particular, there is evidence that heterogeneity in plant communities expressed over space, particularly heterogeneity that induces variation in time by influencing plant phenology, offers fundamentally important nutritional benefits to foraging herbivores that enhance the performance of individuals and their populations (Pettorelli et al 2007;Hebblewhite et al 2008;Mysterud et al 2008, Searle et al 2010Giroux et al 2014, Hurley et al 2014, Iversen et al 2014. For instance, fine scale dynamics of vegetation green-up across landscapes may determine the length of time during spring when high quality forage is available for ungulates.…”

“…This means that access to heterogeneity can be a critically important feature of habitats for mobile herbivores (Owen-Smith 2004;Fryxell et al 2005;Hobbs et al 2008;Searle et al 2010). If access to heterogeneity is limited by habitat fragmentation, mobile herbivores can suffer a reduction in diet quality and food availability leading to deleterious changes to population dynamics and abundance (Hobbs et al 2008, Hobbs & Gordon 2010, Searle et al 2010, Blackburn et al 2011, Herbener et al 2012). These interactions between access to spatial and temporal heterogeneity and ungulate performance will mediate the response of ungulate populations to environmental change, such as changing land-use (including oil and gas development) and climate change.…”

Construction of energy landscapes can clarify the movement and distribution of foraging animals

“…For example, although it is widely accepted that many birds enhance their flight capacities by making use of predictable sources of rising air [5] and that terrestrial animals expend more energy moving over soft substrate than hard [6], general consideration of the energetic costs of animals moving through their variable landscapes is minimal (but see [7]). Landscapes vary in character in both space and time with, for example, heterogeneous vegetation landscapes changing during succession [8,9] and over the growing season [10], becoming correspondingly more problematic for animals to move through [11]. Indeed, the degree of variation in the landscape (e.g.…”

Spatial and Temporal Heterogeneity Creates a “Brown Tide” in Root Phenology and Nutrition

“…This is especially true for spatial heterogeneity that causes temporal variation in plant phenology (e.g., budburst, flowering, and fruiting), because it provides nutritional benefits to herbivores by prolonging the period they can forage on highly nutritious immature plants [3]. Thus, variable landscapes increase the ability of herbivores to access highly nutritious plants by reducing spatial and temporal autocorrelation in plant growth [4]. Without the influence of spatial heterogeneity on plant phenology, plant growth would be synchronized, thereby reducing the time period during which animals could consume the most nutritious plants [5].…”

“…Without the influence of spatial heterogeneity on plant phenology, plant growth would be synchronized, thereby reducing the time period during which animals could consume the most nutritious plants [5]. Access to landscape heterogeneity can therefore enhance population performance of herbivores and may be a critically important feature of their habitat [4,6,7].…”